Radar analysis of warm rain showers¹

1 Part of the “Warm Rain Project”, the results of which were published in Tellus 19, 347-516.

Abstract

Two X-band radar sets, an M-33 and a GPG-1, were operated routinely on the island of Hawaii during the period July 11 through August 28, 1965. The M-33 was located at the Cloud Physics Observatory on the Hilo Campus of the University of Hawaii. The data obtained from the PPI scope photography were used to determine preliminary climatological parameters associated with trade wind showers.

The relationship between echo frequency and total rainfall was examined and found to be remarkably good. Such an analysis is useful in the determination of rainfall amounts over the land and over the ocean in the vicinity of Hawaii where the raingage density is sparse.

The frequency of echoes of a given diameter as a function of range was tabulated according to whether the echo was observed over land or over water. It is apparent from these data that there was a greater frequency of echoes per unit area over land than over water. This is attributed to the breakup of organized lines or bands of showers as they moved inland. In addition, there is a distinct decrease of echo frequency with range. The observed decrease of echo frequency cannot be accounted for by range attenuation or atmospheric refraction.

The relationship between echo movement and the trade winds was studied. The results indicate that the radar echoes tend to move to the right of the wind direction with a slightly greater speed than the environmental wind.

Selected examples of radar photographs are presented which illustrate typical echo displays. In addition, the radar presentation of a mesosystem and the echoes associated with a Hawaiian thunderstorm are shown.

The GPG-1 radar was located at 2100 meters on the windward slope of Mauna Kea. Very little useful data were obtained from this location due to trapping gradients associated with the trade inversion. An example is shown where the propagation path was extended and the sensitivity reduced by refractive index gradients. The site was abandoned early in the project in favor of a more concentrated effort on the M-33 operations.